fix: database target writes to dedicated archive table

The "database" target type now writes events to a separate
archived_events table instead of just marking the delivery as done.
This table persists independently of internal event retention/pruning,
allowing the data to be consumed by external systems or preserved
indefinitely.

New ArchivedEvent model copies the full event payload (method, headers,
body, content_type) along with webhook/entrypoint/event/target IDs.

internal/database/model_archived_event.go

@@ -0,0 +1,19 @@
+package database
+
+// ArchivedEvent stores webhook events delivered via the "database" target type.
+// These records persist independently of internal event retention and pruning,
+// providing a durable archive for downstream consumption.
+type ArchivedEvent struct {
+	BaseModel
+
+	WebhookID    string `gorm:"type:uuid;not null;index" json:"webhook_id"`
+	EntrypointID string `gorm:"type:uuid;not null" json:"entrypoint_id"`
+	EventID      string `gorm:"type:uuid;not null" json:"event_id"`
+	TargetID     string `gorm:"type:uuid;not null" json:"target_id"`
+
+	// Original request data (copied from Event at archive time)
+	Method      string `gorm:"not null" json:"method"`
+	Headers     string `gorm:"type:text" json:"headers"` // JSON
+	Body        string `gorm:"type:text" json:"body"`
+	ContentType string `json:"content_type"`
+}